Hmm, I thought ECM and tap-disintegrating EDM (which is not very precise and does use a huge amount of power, unlike, say, wire EDM) had pretty astonishingly huge MRRs?

I think the density ratio for tungsten and steel (19:8, just over 2:1) is actually more even than the atomic mass ratio (23:7, just over 3:1). So I'd think that even on a volume-per-coulomb basis ECM would cut tungsten faster, assuming comparable faradaic efficiencies. Do you have any idea about the electrode-potential question? I'm totally lost.

No, but if I had a machine like that here I'd definitely give that a work out because this is an interesting question that I normally would not have considered.

That's a question worth settling, from memory the speed is not so much a function of the material as it is of the current density that you can achieve with the gear you have. Thousands of Amps will get you some pretty good speeds, on the order of a cubic cm of material removed per minute for a 25KW machine.

I also wonder what would happen if you tried to plasma cut it, that should work as well.

If there is one thing I really miss from life in Canada then it was the fully equipped machine shop, that was such a great thing to have. You could go from idea to a pretty decent physical implementation fast than you could have ever mail ordered the parts and there is so much satisfaction in working metal. The plasma torch I had there was a relatively small one (12 KW) but it still cut anything an everything I ever threw at it with pretty impressive speed. But I never tried it on Tungsten. Missed chance! I suspect it wouldn't work all that well because Tungsten isn't the best conductor and probably would backsplatter all over the place from the compressed air dispersing the metal that would melt rather than evaporate so you get a puddle rather than a stream of gas.

I don't really understand what the limiting factors on ECM speed are. There's electrolyte heating, cathode heating, bulk electrolyte resistance, formation of salt boundary layers on both the cathode and the anode, and so on. But one thing I do know is that MRR is proportional to the area of the interelectrode process gap: twice the area gives you twice the MRR at the same current density, whether the current density bottleneck is in the cathode, the anode, or the bulk electrolyte between them.

Electrolytic processes are really interesting and complicated, and 200+ years after Davy used them to revolutionize chemistry, I think they're still underused. I'm preetty sure you can produce Fresnel reflectors for a given wavelength, for example, by anodizing aluminum foil to about a quarter-wavelength depth, and across a wide range of visible wavelengths by anodizing it to about 3 microns depth, with the electropolishing effect inherently eliminating the small asperities that make it so slow to produce lenses and mirrors by grinding.

Plasma cutting should work, but should be slower on W than on iron; WP says that at room temperature its specific heat is 24.27 J/mol/K, but at 183.84 g/mol, that's only 0.13 J/g/K. For iron the same figures are 25.10 J/mol/K and 55.845 g/mol, so 0.45 J/g/K. So tungsten should heat up three times as fast with the same power output at room temperature; but tungsten's heat of fusion is four times higher, and I think that's actually the dominant component of plasma-cutting energy consumption. But I think you have roughly four orders of magnitude more knowledge about plasma cutting than I do.

That Fresnel reflector idea sounds super interesting.

Has anybody tried that?

Not that I've heard of; it's possible that someone tried it and it doesn't work for some reason that isn't obvious to me, and then they didn't publish their negative result. I published the idea in 02019 in https://dercuano.github.io/notes/mechano-optical-vector-disp..., but I haven't actually tried it, and not many people read Dercuano, in part because most of it is ideas I thought up but haven't tried.

Reading I've done since then explains that, when anodizing, the anodized layer grows up out of the surface roughly as far as the surface gets depressed, and the anodized layer has a higher refractive index than air, so, for many purposes, you don't need to etch nearly as much metal. (The anodized layer is thin enough that it would be adequate in many applications that normally require a first-surface mirror, though not all.)

It's well known that if you're instead anodizing silicon you can fabricate a rugate filter by varying the anodizing current to vary the density of the anodized layer, that when anodizing titanium you get brilliant iridescence due to the high refractive index of the oxide and consequent strong single-layer dichroic filtering effect, and that there's a negative feedback on the current from the oxide layer thickness in all of these cases, requiring higher voltage to get a thicker oxide; this should also tend to even out small surface asperities, and is not the same effect as anodic leveling. The possibility of including such dichroic filters on the surface of the holographic reflective optics, with wavelength response tailored at a submillimeter or even submicron X-Y resolution, could be interesting for fabricating a wide variety of optical systems.

If you end up trying it, I'd love to hear about the results! If I'm not dead by then. Though I'm sure you have a long list of ideas of your own that you can't wait to try, like everyone else capable of trying this sort of thing.

All this seems to me like a crucial enabling technology for feedback control of machine tools, because light waves don't distort or change their dimensions much when the temperature in the room changes or there's a side load on your gantry.

I need another life ;) Seriously though, this is worth pursuing. But I've more or less decided to dedicate the rest of my life to music and to start this all up again would require a small fortune and a very large dedication in time. Still. Tempting.

Oh, I had no idea! I thought you were doing some VC-fund thing.